Normal fertilization occurs with eggs lacking the integrin α6β1 and is CD9-dependent

Previous results, based on inhibition of fertilization by an anti-alpha 6 integrin mAb (GoH3), suggest that the alpha 6 beta 1 integrin on mouse eggs functions as the receptor for sperm (Almeida, E.A., A.P. Huovila, A.E. Sutherland. L.E. Stephens PG. Calarco, L.M. Shaw A.M. Mercurio, A. Sonnenberg, P. Primakoff, D.G. Myles, and J.M. White. 1995. Cell. 81:1095-1103). Because the egg surface tetraspanin CD9 is essential for gamete fusion (Kaji, K., S. Oda, T. Shikano, T. Ohnuki, Y. Uematsu. J. Sakagami, N. Tada, S. Miyazaki, and A. Kudo. 2000. Nat. Genet. 24:279-282; Le Naour. F.,E. Rubinstein, C. Jasmin, M. Prenant, and C. Boucheix. 2000. Science. 287:319-321; Miyado, Ii., G. Yamada, S. Yamada, H. Hasuwa, Y. Nakamura, F. Ryu Ii. Suzuki, K. Kosai, K. Inoue, A. Ogura, M. Okabe, and E. Mekada. 2000. Science. 287:321-324) and CD9 is known to associate with integrins, recent models of gamete fusion have posited that egg CD9 acts in association with alpha 6 beta 1 in fusion (Chen, M.S., K.S. Tung, S.A. Coonrod, Y. Takahashi, D. Bigler, A. Chang, Y. Yamashita, P.W. Kincade, J.C. Herr, and J.M. White. 1999. Proc. Natl. Acad. Sci. USA. 96:11830-11835, Kaji, K., S. Oda, T. Shikano, T. Ohnuki, Y. Uematsu, J. Sakagami, N. Tada, S. Miyazaki, and A. Kudo. 2000. Nat. Genet. 24:279-282; Le Naour, E, E. Rubinstein, C. Jasmin, M. Prenant, and C. Boucheix. 2000. Science. 287:319-321; Miyado, K., G. Yamada, S. Yamada, H. Hasuwa, Y. Nakamura. F. Ryu, K. Suzuki, K. Kosai, K. Inoue, A. Ogura, M. Okabe, and E. Mekada. 2000. Science. 287:321-324). Using eggs from cultured ovaries of mice lacking the alpha 6 integrin subunit, we found that the fertilization rate, fertilization index, and sperm binding were not impaired compared with wild-type or heterozygous controls. Furthermore, a reexamination of antibody inhibition, using an assay that better simulates in vivo fertilization conditions, revealed no inhibition of fusion by the GoH3 mAb. We also found that an anti-CD9 mAb completely blocks sperm fusion with either wild-type eggs or eggs lacking alpha 6 beta 1. Based on these results, we conclude that the alpha 6 beta 1 integrin is not essential for sperm-egg fusion, and we suggest a new model in which CD9 acts by itself, or interacts with egg protein(s) other than alpha 6 beta 1,to function in sperm-egg fusion.